Coverage of "Britain's fattest teenager" has swamped the press – with pictures of the fire crews used to demolish the 19-year-old's house and winch her into a specially adapted obesity ambulance after she collapsed with breathing problems. Unsurprisingly, it has also spawned a slew of unhelpful commentary denigrating fat people for their laziness, gluttony and all-round uselessness.

It's rare for someone not to think before making a remark that could cause offence about race, religion, sexual orientation or even age. Our thoughts are our own, but our words are usually chosen with more care. But someone's size, shape and weight seems to be the last domain where a pejorative remark can be produced unchallenged. On a BBC radio phone-in about the news story a caller said: "I hate it when I see their fat bodies wobbling towards me." It's not an uncommon reaction, but it is an unacceptable one.

Our charity, Beat has been campaigning for 22 years to raise awareness of eating disorders – anorexia and bulimia – and we have seen a welcome increase in more compassionate reporting. There is more understanding now that these are serious mental illnesses that can affect anyone at any age and not – as was once assumed to be the case – the choice of silly little rich girls trying to seek attention or look like their catwalk idols. Anorexia is the rarest eating disorder, accounting for just 10% of cases. Most people with an eating disorder will be at a "healthy" weight – their pain and distress not visible in any protruding bones.

So it's sad that we have yet to see the same compassion extended to people who are, for whatever reason, overweight. Obesity isn't an illness – although it can cause ill health, both physical and mental – but it is rarely a matter of choice either. Our biology plays a significant role driving us to choose the most calorie-dense foods available. Rats, given the choice of as much "healthy" rat food as they want freely available, or access to rat "junk" they can only get by taking shocks to their feet, go for the shock factor every time to get the fatty, sugary stuff. They don't see TV or have body image issues – just a biological mechanism designed to survive famine – much like our own.

When someone calls the Beat helplines they are usually at a point of crisis or extreme distress. We listen daily to people who are ashamed of themselves, disgusted with their bodies, and know that their relationship with food is all wrong. What we don't know is what size they are. Their experiences, their feelings about themselves have so much in common whether they are emaciated or obese. It's the emotional relationship with food that has got out of sync with our essential biology.

Naming, shaming and blaming people for their weight, shape and size doesn't work with anorexia and won't work with obesity either. It's a complex issue – a toxic obesogenic environment laden with piled-high processed food literally as cheap as chips – while at the same time thinness and the self-control it takes to achieve is highly prized.

We know that people with the serious mental illness of eating disorders have come forward in ever greater numbers for the treatment and help they need because the shame and stigma of the condition has been challenged head-on.

Let's stop fat shaming now – it serves no purpose. The only shame is on the person who still thinks it's acceptable to show their prejudice about weight.